U.S. patent application number 16/961347 was filed with the patent office on 2020-11-05 for method for producing moulded parts consisting of a porous material impregnated with polysulfide.
The applicant listed for this patent is BASF SE. Invention is credited to Domnik Bayer, Wolfgang Jabczynski, Johan ter Maat, Jesus Enrique Zerpa Unda.
Application Number | 20200350552 16/961347 |
Document ID | / |
Family ID | 1000005018056 |
Filed Date | 2020-11-05 |
United States Patent
Application |
20200350552 |
Kind Code |
A1 |
Bayer; Domnik ; et
al. |
November 5, 2020 |
METHOD FOR PRODUCING MOULDED PARTS CONSISTING OF A POROUS MATERIAL
IMPREGNATED WITH POLYSULFIDE
Abstract
Described herein is a process for the production of moldings
made of porous material impregnated with polysulfide, the process
including the following steps: (a) insertion of the porous material
into a mold; (b) introduction of liquid polysulfide into the mold
at a flow rate within the porous material in the range from 0.5 to
200 cm/s; (c) cooling of the polysulfide to a temperature below the
melting point of the polysulfide; and (d) removal of the porous
material impregnated with the polysulfide.
Inventors: |
Bayer; Domnik;
(Ludwigshafen, DE) ; Zerpa Unda; Jesus Enrique;
(Ludwigshafen, DE) ; Jabczynski; Wolfgang;
(Ludwigshafen, DE) ; ter Maat; Johan;
(Ludwigshafen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BASF SE |
Ludwigshafen am Rhein |
|
DE |
|
|
Family ID: |
1000005018056 |
Appl. No.: |
16/961347 |
Filed: |
January 10, 2019 |
PCT Filed: |
January 10, 2019 |
PCT NO: |
PCT/EP2019/050550 |
371 Date: |
July 10, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B28B 7/42 20130101; H01M
4/0485 20130101; H01M 10/3909 20130101; H01M 4/5815 20130101; H01M
4/0433 20130101 |
International
Class: |
H01M 4/04 20060101
H01M004/04; H01M 4/58 20060101 H01M004/58; H01M 10/39 20060101
H01M010/39 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 16, 2018 |
EP |
18151853.1 |
Claims
1. A process for the production of moldings made of porous material
impregnated with polysulfide, comprising the following steps: (a)
insertion of the porous material into a mold; (b) introduction of
liquid polysulfide into the mold at a flow rate within the porous
material in the range from 0.5 to 200 cm/s; (c) cooling of the
polysulfide to a temperature below the melting point of the
polysulfide; and (d) removal of the porous material impregnated
with the polysulfide.
2. The process according to claim 1, wherein after the insertion of
the porous material the mold is flushed with an inert gas.
3. The process according to claim 1, wherein, before the
introduction of the polysulfide, a pressure below 100 mbar (abs) is
applied to the mold.
4. The process according to claim 1, wherein the temperature of the
mold is in the range from 150 to 350.degree. C.
5. The process according to claim 1, wherein the temperature of the
polysulfide when it is introduced into the mold is in the range
from 285 to 350.degree. C.
6. The process according to claim 1, wherein, after the material
has been introduced into the mold, a pressure in the range from 150
to 200 bar is used to introduce further polysulfide.
7. The process according to claim 1, wherein introduction of the
polysulfide into the mold is achieved by using a cold-chamber
process, a hot-chamber process or a vacuum casting process.
8. The process according to claim 1, wherein the polysulfide is an
alkali metal polysulfide.
9. The process according to claim 8, wherein the polysulfide is
sodium polysulfide.
10. The process according to claim 1, wherein the porous material
is a felt, a woven fabric, a knitted fabric, a braided fabric, a
nonwoven fabric, an open-pore foam or a three-dimensional
network.
11. The process according to claim 1, wherein the porous material
is composed of graphite, of heat-stabilized polymer fibers, of
fibers made of oxide ceramic, of glass fibers or of a mixture
thereof.
12. The process according to claim 1, wherein the molding is an
electrode or a part of an electrode for an electrochemical
cell.
13. The process according to claim 1, wherein the molding is a
storage element for use in an electrochemical cell.
14. The process according to claim 12, wherein the electrochemical
cell is a sodium-sulfur battery.
Description
[0001] The invention relates to a process for the production of
moldings made of porous material impregnated with polysulfide.
[0002] These polysulfide-impregnated moldings are used by way of
example as electrodes or electrode parts, or else as storage
elements for anode material in the production of electrochemical
cells, in particular of sodium-sulfur batteries.
[0003] The storage capacity of batteries depends on the quantity
present of the reactants used, and additional containers are
therefore used in order to increase storage capacity. In
sodium-sulfur batteries, discharge is achieved by using the liquid
sodium alongside a solid electrolyte that is permeable to cations.
The liquid sodium simultaneously serves as anode and forms cations
which are transported to the cathode via the cation-conducting
solid electrolyte. At the cathode, the sulfur flowing toward the
cathode is reduced to polysulfide, i.e. is reacted with sodium ions
to give sodium polysulfide. This sodium polysulfide can be
collected in another container. Alternatively, it is also possible
to collect the sodium polysulfide together with the sulfur in the
container around the cathode chamber. Because of the density
difference, the sulfur rises and the sodium polysulfide settles.
This density difference can also be utilized to bring about flow
along the cathode. A corresponding battery design is described by
way of example in WO-A 2011/161072 or WO-A 2017/102697.
[0004] A general problem in the production of sodium-sulfur
batteries is the introduction of the reactants used. In order to
permit problem-free operation of the battery, it is necessary to
achieve complete sulfur-impregnation of the porous material which
surrounds the solid electrolyte and is usually used as cathode. In
order to ensure uniform transport of the sulfur during the usual
operation of the battery, there is additionally a porous material,
usually a felt or a nonwoven fabric, introduced into the space
surrounding the electrode and serving as sulfur reservoir. The
procedure generally used to introduce materials into the battery
introduces liquid sodium into the sodium container and utilizes
sulfur-saturated porous electrodes. Processes for the production of
sulfur-saturated porous electrodes are described by way of example
in JP-A 2004082461, KR-A1020130075495 or KR-A 1020140085758.
[0005] A particularly problematic aspect of the introduction of
alkali metal and sulfur is the handling of the highly reactive
liquid alkali metal, in particular of sodium. WO-A 2017/102697
discloses, with the aim of permitting less dangerous and
problematic introduction of materials into the battery, use of
polysulfide to impregnate the porous electrode and the porous
sulfur-reservoir material, and insertion of the resultant
impregnated porous electrode and the impregnated reservoir material
into the electrochemical cell. It is then no longer necessary to
introduce liquid alkali metal, in particular liquid sodium. In
order to convert the electrochemical cell to a condition that is
ready for use, an initial post-assembly charging cycle is then
carried out in which the alkali metal polysulfide is cleaved to
give alkali metal and sulfur, where the alkali metal ions pass
through the solid electrolyte, are neutralized by absorbing
electrons at the electrode in the anode chamber and are collected
and stored in the alkali metal container.
[0006] The processes used for a sulfur melt to impregnate the
porous materials for the electrode and the reservoir material
cannot be used for impregnation with the alkali metal polysulfide,
because the properties of the alkali metal polysulfide differ from
those of sulfur, in particular in that the former has a higher
melting point and, at this temperature, a relatively low viscosity.
Furthermore, because the alkali metal polysulfide melt can
self-ignite, contact with oxygen must be prevented during the
production of the polysulfide-impregnated porous materials, as long
as the polysulfide is present in liquid form. Alkali metal
polysulfide is moreover hygroscopic, and contact thereof with
water, including the water present in the form of atmospheric
humidity, must be minimized. It has moreover been found that when
high pressure is used to introduce the materials, the structure of
the porous material can be altered, or even damaged, by the ingress
of the polysulfide.
[0007] It was therefore an object of the present invention to
provide a process which can produce moldings made of porous
material impregnated with polysulfide and which can be operated
safely, and without damage to the structure of the porous
material.
[0008] The object is achieved via a process for the production of
moldings made of porous material impregnated with polysulfide,
comprising the following steps:
[0009] (a) insertion of the porous material into a mold;
[0010] (b) introduction of liquid polysulfide into the mold at a
flow rate within the porous material in the range from 0.5 to 200
cm/s;
[0011] (c) cooling of the polysulfide to a temperature below the
melting point of the polysulfide;
[0012] (d) removal of the porous material impregnated with the
polysulfide.
[0013] Surprisingly, it has been found that when the flow rate of
the polysulfide in the porous material is in the range from 0.5 to
200 cm/s, preferably in the range from 0.5 to 50 cm/s, saturation
of the porous material is achieved and the structure of the porous
material is not changed in any manner that then prevents operation
of the electrochemical cell without performance losses. That type
of change occurs when flow rate is too high, for example through
displacement of fibers in a felt or nonwoven fabric that is used as
porous material. The displacement changes the pore size within the
felt or nonwoven fabric, and this leads to a reduced capillary
action, with the result that during discharge there is then
insufficient polysulfide transported away from the solid
electrolyte and insufficient sulfur transported to the solid
electrolyte, and during charging there is insufficient polysulfide
transported to the solid electrolyte and insufficient sulfur
transported away from the solid electrolyte. Optimal operation of
the electrochemical cell requires that during discharge the solid
electrolyte is always ideally entirely wetted by sulfur and that
the polysulfide is transported away from the solid electrolyte
immediately after the reaction of alkali metal with sulfur, and
correspondingly requires during charging that the solid electrolyte
is always entirely wetted by polysulfide and that the sulfur is
transported away from the solid electrolyte immediately after
cleavage of the polysulfide to give sulfur and alkali metal. This
transport is assisted by the capillary action of the pores in the
porous electrode and corresponding wettability of the porous
material by polysulfide and, respectively, sulfur.
[0014] For the purposes of the present invention, "polysulfide"
means the reaction product X.sub.yS.sub.n of anode material X,
usually an alkali metal, in particular sodium, and sulfur. n is a
number from 1 to 5.2, and gives the average length of the sulfur
chains present in the polysulfide; the polysulfide here is
frequently a mixture of polysulfides, each with a different number
of sulfur atoms per molecule; y is an integer selected in a manner
such that the reaction product of anode material and sulfur is
electrically neutral. Because the sulfur chains always have a
charge of minus two, y=2 by way of example for an alkali metal as
anode material X.
[0015] In a first step (a) for the production of the porous
material impregnated with polysulfide, the porous material is
inserted into the mold. The mold here is a conventional mold as
used in injection processes or casting processes, and has an
interior surface shape corresponding to the exterior shape of the
required molding. Because the polysulfide is highly corrosive, all
of the mold surfaces that come into contact with polysulfide are
made of a material that is inert with respect to the polysulfide.
To this end, it is possible to coat the surfaces of the mold or
alternatively to manufacture the mold from a material that is inert
with respect to the polysulfide and resistant to the temperatures
arising. Examples of suitable materials for a coating or for the
production of the mold are chromium, glass and ceramics. It is
therefore possible by way of example to manufacture the mold from
cast iron or from steel, for example from stainless steel or from
hardened tool steel, and to provide a chrome coating to same. The
chrome coating here is preferably applied electrochemically, i.e.
in an electroplating process. If glass or ceramic is used as
material for the mold, it is preferable that the latter is
manufactured entirely from these materials. Alternatively, it is
also possible to apply a thin ceramic coating, for example by a
gas-phase deposition process such as CVD. For better demolding of
the parts produced, the surfaces that form the shape of the molding
are advantageously polished surfaces.
[0016] Once the porous material has been inserted, the mold is
closed. In order to facilitate the subsequent assembly of the
electrochemical cell when the porous material impregnated with
polysulfide is used in an electrochemical cell, the internal shape
of the mold is designed so that the dimensions of the
polysulfide-impregnated porous material produced in the mold permit
easy insertion into the electrochemical cell. This means that there
is always a small distance between the external surfaces of the
resultant polysulfide-impregnated porous material and surfaces with
which the impregnated porous material is in contact in the
assembled condition of the electrochemical cell. In order that
during operation of the electrochemical cell the porous material is
in contact with the respective external surfaces, it is moreover
preferable that the porous material is somewhat compressed in the
mold. After incorporation in the electrochemical cell and melting
of the polysulfide, the porous material then expands and comes into
contact with the respective external surfaces of the
electrochemical cell. The shape of the resultant
polysulfide-impregnated porous material, and the compression of the
porous material on insertion into the mold, are selected here in a
manner that ensures that after relaxation of stresses in the
electrochemical cell the porous material is in contact with all
contact surfaces in the electrochemical cell. It is therefore
preferable that, even after the reversal of deformation in the
electrochemical cell, the porous material retains a small degree of
compression.
[0017] The porous material is compressed in the mold to a volume
that is dependent on the use of the porous material in the
electrochemical cell and is smaller than the volume of the
uncompressed porous material by from 0 to 50%, preferably from 5 to
40% and in particular from 5 to 30%. When the porous material is
used as electrode part, it is preferable that the porous material
is compressed in the mold to a volume that is smaller than the
volume of the uncompressed porous material by from 5 to 50%, more
preferably from 10 to 40% and in particular from 20 to 30%. When
the porous material is used for reservoir parts, compression is
less important, and therefore in this case it is preferable that
the porous material is compressed to a volume that is smaller than
the volume of the uncompressed porous material by from 0 to 30%,
more preferably from 5 to 20% and in particular from 5 to 15%.
[0018] Molten polysulfides tend to self-ignite in the presence of
oxygen, and it is therefore advantageous that, after insertion of
the porous material and after closing, the mold is flushed with an
inert gas. Inert gas used here can be any gas that does not react
with the polysulfide. Suitable inert gases are in particular
nitrogen, carbon dioxide and noble gases. Nitrogen is particularly
preferred as inert gas. The flushing with the inert gas removes the
gas previously present in the mold, generally air. The oxygen
present in the mold is thus also removed, and there is therefore no
risk of self-ignition when the polysulfide is introduced into the
mold. Additionally or alternatively, it is also possible to apply a
reduced pressure in order to remove the air and thus the oxygen
from the mold. It is preferable for this purpose to apply a
pressure below 100 mbar (abs) to the mold before the polysulfide is
introduced. Application of a reduced pressure here has the
additional advantage that the transport of the polysulfide into the
mold is thus assisted and in particular complete filling of the
mold is also permitted, because less pressure is required to
compress the gas present in the mold in order to introduce the
polysulfide into the mold. It is particularly preferable that the
reduced pressure applied to the mold is below 80 mbar (abs) and in
particular below 60 mbar (abs).
[0019] In order to prevent solidification of the polysulfide on the
mold wall during introduction into the mold and entrainment of
solidified polysulfide with the melt, with resultant possible
damage to the porous material and thus production of defective
moldings, the temperature of the mold is preferably in the range
from 150 to 350.degree. C. A temperature in this range is
sufficient to prevent solidification of polysulfide during
introduction into the mold. If, by way of example, a metal casting
mold is used for the production of the polysulfide-impregnated
porous material, it is possible that this is heated to a
temperature above the melting point of the polyamide and is cooled
after the material has been introduced.
[0020] After the polysulfide has been introduced it is cooled to a
temperature below the melting point. In order to avoid shrinkage
and thus to produce moldings corresponding to the desired geometric
requirements, it is preferable that, during cooling, further
polysulfide is introduced under pressure into the mold. In order to
permit the polysulfide in the mold to solidify, it is necessary
that the temperature of the mold is below the melting point of the
polysulfide. On the other hand, however, in order to avoid
excessively rapid solidification of the polysulfide, which would
inhibit or prevent introduction of further material intended to
ensure that the shape of the molding is as desired, or which could
cause damage to the porous material, it is also necessary that the
temperature of the mold is not excessively low. A mold temperature
in the range from 150 to 350.degree. C. is also advantageous for
introducing further material in order to achieve the desired
shape.
[0021] The mold temperature here means the temperature at the
surfaces that come into contact with the polysulfide. An example of
a method for controlling the temperature of the mold is provision
of electrical heating or alternatively formation, in the mold, of
channels through which a temperature-control medium flows, for
example steam or hot fluid.
[0022] In order both to avoid excessively rapid solidification
during the introduction of the polysulfide into the mold and also
to permit introduction of further material to order to achieve the
desired shape, the mold temperature, i.e. the temperature of the
mold surfaces coming into contact with the polysulfide, is
preferably in the range from 150 to 350.degree. C., more preferably
in the range from 150 to 250.degree. C. and in particular in the
range from 170 to 230.degree. C.
[0023] In order to prevent further deformation of the resultant
molding made of polysulfide-impregnated porous material after
removal from the mold, all of the polysulfide introduced into the
mold has advantageously solidified before the mold is opened for
the removal of the molding. This is achieved in that the mold
remains closed for from 10 to 300 s after the introduction of the
polysulfide, preferably from 30 to 180 s, in particular from 60 to
120 s. This time in particular depends on the temperature of the
mold, the temperature of the melt, and the size of the resultant
molding made of polysulfide-impregnated porous material. The higher
the temperature of the mold and the larger the molding, the longer
the required retention time of the molding in the mold after
introduction of the polysulfide into the mold has been concluded
and before the mold can be opened to remove the molding.
[0024] Contact of the polysulfide in the molten state with oxygen
must be avoided, because the polysulfide can self-ignite, and it is
therefore preferable that the polysulfide in solid form is
introduced into a feed container. After introduction of the
polysulfide, said container is inertized, for example by flushing
with an inert gas. Inert gases that can be used here are the gases
described above for the flushing of the mold. It is preferable to
use argon to inertize the container. Alternatively or additionally,
it is also possible to evacuate the container after the polysulfide
has been introduced and after it has been closed. After
inertization, the container is heated to a temperature above the
melting point of the polysulfide, so that the polysulfide in the
container melts. If the polysulfide is sodium polysulfide as is
usually used in electrochemical cells operating on the basis of
sodium as anode material and sulfur as cathode material, the
container is heated to a temperature above 285.degree. C.,
preferably to a temperature of from 285 to 350.degree. C., so that
the temperature of the polysulfide after melting is likewise in the
range from 285 to 350.degree. C. In order to accelerate the melting
of the polysulfide, the temperature during the melting procedure is
advantageously above 285.degree. C. The greater the temperature
difference between the melting point of the polyamide and the
temperature of the container, the greater the heat flux that can be
introduced to melt the polysulfide.
[0025] The feed container is then connected to the mold by way of a
suitable connecting line. Before the connection from the feed
container into the connecting line is opened, for example via a
valve at the outlet of the feed container, it is advantageous to
inertize the connecting line, for example as described above by
flushing with an inert gas and/or by evacuation. In this type of
setup where the feed container is connected to the mold by way of a
connecting line, there is the possibility of easy change of the
feed container once the polysulfide has been consumed. In this case
it is possible to melt the polysulfide in a feed container not
connected to the mold, and a feed container filled with molten
polysulfide is attached to the connecting line. This permits
production of the moldings made of polysulfide-filled porous
material without the long interruptions which would be required if,
on each occasion after the polysulfide had been consumed, it were
necessary to introduce fresh polysulfide into the feed container,
inertize same, and then melt the polysulfide. Alternatively, it is
also possible to introduce further polysulfide regularly into the
feed container, or to provide an inertized container in which solid
polysulfide has previously been introduced and, with use of
metering equipment, for example a rotary valve, connect said
container to the feed container for the molten polysulfide, and
thus meter further solid polysulfide into the feed container for
the liquid polysulfide either when a lower threshold quantity is
reached or else alternatively continuously; the solid polysulfide
in the feed container here is melted in the liquid polysulfide
already present therein.
[0026] For the possibility of changeover of the feed container, it
is advantageous to leave polysulfide in the connecting line. This
allows operation to continue without fresh inertization of the
connecting line after the changeover of the feed container. Because
in this case it is necessary to close the ends of the connecting
line on account of the risk of self-ignition, the only requirement
is then to inertize the part between the valve at the outlet of the
feed container and the valve on the connecting line at the end at
which the connecting line is attached to the feed container. In
order to ensure that the polysulfide cannot solidify in the
connecting line, it is preferable to provide trace heating to the
latter, an example being a heating wire, or a jacket through which
a temperature-control medium flows. It is also necessary to
insulate the connecting line in order to avoid overheating.
[0027] The polysulfide is introduced in the form of melt into the
mold. The melting point is by way of example in the range from 235
to 285.degree. C. when sodium polysulfide is used. The temperature
at which the sodium polysulfide is introduced into the mold is
preferably in the range from 285 to 350.degree. C. and in
particular in the range from 300 to 330.degree. C.
[0028] Because the polysulfide shrinks during solidification, it is
necessary during cooling to force further polysulfide into the
mold. The magnitude of the pressure used for this purpose must also
be sufficient to cause onward movement, at locations of the molding
that are remote from the point of injection, of polysulfide used
for shrinkage-compensation. This permits production of moldings of
precisely correct shape. This is in particular necessary in order
to permit easy assembly of the electrochemical cell. Because the
requirement to avoid deformation of the porous material also
applies during the introduction of further material, the pressure
used to introduce the polysulfide after filling of the mold is
preferably in the range from 150 to 200 bar. This pressure allows
production of moldings of precisely correct shape without
deformation of the porous material.
[0029] Various casting processes can be used to introduce the
polysulfide into the mold. It is preferable that the polysulfide is
introduced into the mold by a cold-chamber process, a hot-chamber
process or a vacuum casting process.
[0030] In the case of the cold-chamber process, the polysulfide is
heated in an oven and melted, conveyed from the oven into a
conveying unit which can generate a superatmospheric pressure in
the mold, and forced by the conveying unit into the mold. The
hot-chamber process differs from this in that the conveying unit
which forces the polysulfide into the mold is part of the oven. In
particular, a piston unit is used as conveying unit which can
generate the superatmospheric pressure in the mold. This type of
piston unit comprises a compression chamber into which the
polysulfide flows from the feed container or from the oven, and a
piston which, in a subsequent step, forces the polysulfide from the
compression chamber into the mold. However, it is also possible to
use any other suitable conveying unit as alternative to this type
of piston unit, an example being a pump or else a
reciprocating-screw machine. However, particular preference is
given to a piston unit.
[0031] The vacuum casting process differs from the cold-chamber
process and hot-chamber process in that the polysulfide is not
forced into the mold by a conveying unit. A reduced pressure is
applied at the mold and the molten polysulfide flows into the mold
by virtue of the reduced pressure applied at same.
[0032] It is particularly preferable to combine the vacuum casting
process with the hot-chamber process or with the cold-chamber
process. Application of the reduced pressure in particular avoids
the possibility of inclusion of gas into the molding, leading to
incomplete impregnation of the porous material, or of surface
deformation caused by gas bubbles in the mold, because the gas is
removed from the mold before the polysulfide is introduced. An
example of a possibility here is that initially polysulfide is
introduced into the mold solely by virtue of the reduced pressure
applied, and that the conveying unit is used only for the
introduction of further material, but another possibility is that
the conveying unit is used from the beginning to assist
introduction of the polysulfide into the mold. In every case,
however, it is necessary to ensure that the flow rate of the
polysulfide in the porous material is in the range from 0.5 to 200
cm/s, preferably in the range from 0.5 to 50 cm/s and in particular
in the range from 1 to 10 cm/s.
[0033] The molding made of polysulfide-impregnated porous material
is in particular an electrode or part of an electrode for an
electrochemical cell, or else a storage element for use in an
electrochemical cell, in particular for a sodium-sulfur
battery.
[0034] An electrochemical cell in which the moldings made of
polysulfide-impregnated porous material can be used generally
comprises a cathode chamber intended to hold a liquid cathode
material and an anode chamber intended to hold a liquid anode
material, where a solid electrolyte separates the cathode chamber
from the anode chamber and the solid electrolyte is enclosed by a
sheet material with apertures through which the cathode material
can flow. The sheet material has been manufactured from an
electrically conductive material, and the cathode chamber comprises
at least one segment, where each segment has a jacket made of an
electrically conductive material and the jacket has been secured on
the sheet material with apertures in a manner that provides
electrical conductivity. If the cathode chamber comprises more than
one segment, the connection of the jacket to the sheet material
with apertures must moreover be impermeable to fluids. The
respective segments serve as reservoir for the cathode material
and, in order to increase the storage capacity of the
electrochemical cell, it is possible to increase either the number
of segments or the cross-sectional area, and thus the volume, of
the segments. In order that the cathode material is conducted
uniformly to the solid electrolyte and that the reaction product
formed at the solid electrolyte from anode material and cathode
material is conducted away from the solid electrolyte, a porous
material is inserted into the segments. Capillary forces in the
pores of the porous material assist transport of the cathode
material and, respectively, of the reaction product. In order to
avoid handling of the reactive starting materials, in particular
highly reactive alkali metal used as anode material, during
production of the electrochemical cell, the porous material is
impregnated with the polysulfide and then, in the form of molding
made of polysulfide-impregnated porous material, is inserted into
the segments. A corresponding electrochemical cell and production
thereof are described by way of example in WO-A 2017/102697. By
virtue of the use of the moldings made of polysulfide-impregnated
porous material, instead of introduction of cathode material and of
anode material, the electrochemical cell is in discharged condition
after assembly. In order to permit use of the electrochemical cell,
it is therefore first necessary to charge same. To this end, the
electrochemical cell is first heated so that the polysulfide melts.
An electrical current is then applied to the electrochemical cell,
and the polysulfide is thus cleaved at the solid electrolyte to
give cathode material and anode material. The anode material passes
through the solid electrolyte and is collected in the anode
chamber. The cathode material remains in the porous material in the
cathode chamber.
[0035] It is also possible, alternatively or additionally, to
provide an electrode made of a porous material, the arrangement of
this electrode being on that side of the solid electrolyte that
faces toward the cathode chamber. It is preferable here that the
electrode made of porous material encloses the solid
electrolyte.
[0036] When the molding made of polysulfide-impregnated porous
material is used in an electrochemical cell, it is particularly
preferable to use, as polysulfide, an alkali metal polysulfide. It
is particular preferable that the alkali metal polysulfide is
sodium polysulfide.
[0037] The material for the porous material is selected as required
by the function of the molding made of polysulfide-impregnated
porous material. If the porous material is intended for use as
porous electrode in the electrochemical cell, the material used for
the porous material is one that is chemically inert and
electrically conductive and that can be wetted both by sulfur and
by polysulfide. The porous material in this case is preferably
composed of carbon, in particular in the form of graphite.
[0038] In a possible method for improving the transport of material
in the porous electrode it is possible to use, in addition to the
chemically inert and electrically conductive material that can be
wetted by the cathode material, a second material, which does not
necessarily have to be electrically conductive, but which can
easily be wetted by the reaction product of cathode material and
anode material. Particularly suitable materials that can easily be
wetted by the reaction product of cathode material and anode
material are oxide ceramics or glasses, for example aluminum oxide
(Al.sub.2O.sub.3), silicon dioxide, for example glass fibers, mixed
oxides of aluminum with silicon, with silicates and with
aluminosilicates, and also zirconium oxide and mixtures of these
materials. When a material that can easily be wetted by the
reaction product of anode material and cathode material is also
present, the proportion, in the electrode, of the material that can
easily be wetted by the reaction product of cathode material and
anode material is preferably less than 50% by volume, particularly
preferably less than 40% by volume and at least 5% by volume. It is
moreover possible to use a thermal treatment to render the
electrode wettable for the reaction product of anode material and
cathode material. A thermal treatment can by way of example be
carried out at 600.degree. C. in air for from 20 to 240
minutes.
[0039] When the molding made of polysulfide-impregnated porous
material is used as storage material in the cathode chamber, the
porous material is preferably one manufactured from a material that
can easily be wetted by the cathode material and by the reaction
product of cathode material and anode material. In order to obtain
good wetting of the porous material even when cathode material and
reaction product have different wetting properties, it is
advantageous to manufacture the porous material from different
materials, where a portion of the material can easily be wetted by
the cathode material and a portion can easily be wetted by the
anode material. When a mixture of a plurality of different
materials is used for the porous material, it is preferable that
each of these is used in the same proportion by volume. However, it
is also possible to set other ratios by volume as required by the
design of the electrochemical cell. When an alkali metal is used as
anode material and sulfur is used as cathode material, suitable
materials of which the porous material is composed are in
particular thermally stabilized polymer fibers, fibers made of
oxide ceramics or glass fibers, preferably thermally stabilized
polymer fibers in a mixture with fibers made of oxide ceramics or
glass fibers. Suitable fibers made of oxide ceramics or glass
fibers are in particular fibers made of aluminum oxide
(Al.sub.2O.sub.3), silicon dioxide, for example glass fibers, mixed
oxides of aluminum with silicon, with silicates or with
aluminosilicates, zirconium oxide and mixtures of these materials.
Examples of suitable thermally stabilized polymer fibers are
oxidized, thermally stabilized polyacrylonitrile (PAN) fibers which
are available commercially by way of example as PANOX.RTM.. In
order to render these fibers more resistant to temperatures above
300.degree. C., the polymer fibers in a mixture with the oxide
ceramic fibers or glass fibers are advantageously posttreated by
way of example for from 12 to 36 hours at temperatures in the range
from 400 to 500.degree. C. in an inert atmosphere, for example
nitrogen or noble gas such as argon.
[0040] In order that uniform transport of cathode material and
reaction product of cathode material and anode material toward the
electrode and away from the electrode can be ensured when the
porous material is used in an electrochemical cell, in particular
in a sodium-sulfur battery, it is preferable that the porous
material is a felt, a woven fabric, a knitted fabric, a braided
fabric, a nonwoven fabric, an open-pore foam or a three-dimensional
network. If the porous material is a felt, a preferential direction
is provided thereto in order to improve mass transport during the
operation of the electrochemical cell. The preferential direction
can in particular be produced here via needling. The preferential
direction here preferably runs perpendicularly in relation to the
solid electrolyte after assembly of the electrochemical cell.
Alternatively or additionally, it is possible to provide
channel-type structures in order to improve mass transport. These
likewise preferably run perpendicularly in relation to the solid
electrolyte in an assembled electrochemical cell.
[0041] In order, when a porous material has a preferential
direction, to make good use of the preferential direction during
the production of the molding made of polysulfide and of porous
material, and in order to facilitate impregnation with the
polysulfide, the arrangement of the injection point by way of which
the polysulfide is introduced into the mold is advantageously such
that when the polysulfide is introduced into the mold it flows
along the preferential direction in the porous material.
Deformation of the porous material during the introduction of the
polysulfide into the mold is thus in particular further reduced or
prevented.
[0042] Once the polysulfide has been introduced, this is cooled
until it solidifies. The molding made of polysulfide-impregnated
porous material is thus formed. The molding is also dimensionally
stable by virtue of the solidified polysulfide, and therefore
cannot be deformed and thus can easily be used in the production of
the electrochemical cell. Easy deformation of the porous material
is possible again only when the polysulfide has been remelted.
Another advantage of production from polysulfide-impregnated porous
material is that the molding can be handled safely, because the
polysulfide is susceptible to self-ignition only in the molten
state, and no self-ignition takes place when the polysulfide is in
solid form, even in the presence of oxygen.
[0043] In order to achieve safe production of a plurality of
moldings made of polysulfide-impregnated porous material, it is
therefore necessary, after opening of the mold and removal of the
molding, and after the mold has again been closed, to re-inertize
the mold before the polysulfide is introduced. As long as, before
the polysulfide is introduced, the mold is inertized as described
above, for example by flushing with an inert gas or, alternatively
or additionally, by applying a reduced pressure, the process can
also be operated safely for production of a plurality of
moldings.
[0044] Another possibility alongside the use of mold systems having
only one mold to produce a molding is use of mold systems having a
plurality of molds each connected to one another via flow channels,
or connected by way of a distribution system to a central
polysulfide supply, the aim being thus to permit simultaneous
production, in one mold system, of a plurality of moldings made of
polysulfide-impregnated porous material. It is preferable in this
case to connect the individual molds of the mold system by way of a
distribution system with a central supply, so that polysulfide is
simultaneously introduced via the central supply system, by way of
the distribution system, into the individual molds.
* * * * *